We meet Primitivo on the first night, when he, Andrés, and Eladio arrive in the cave while Robert Jordan's away at the bridge; presumably, Pablo tells them all about the bridge plan, and why it sucks.
When Robert Jordan returns, Primitivo sides with Pilar in favor of the bridge.
The next day, Primitivo is presumably on sentry duty.
That night, Primitivo tries to make polite conversation and ignore Pablo's irritating remarks. He succeeds for a little while, but then a confrontation between Pablo and Robert Jordan ensues.
Once Pablo leaves the cave, Primitivo says he is in favor of killing him.
The next day, Primitivo accompanies Robert Jordan, Agustín, and Anselmo to the camp's upper sentry post to take care of any fascists who wander too close to the cave. He is posted a little further up the mountain side, so he can alert the others when a fascist patrol comes in sight. A fascist cavalry patrol does pass by, but does not proceed to the cave.
Primitivo is the first one to notice that El Sordo is being attacked. He wants to go help him, but Robert Jordan refuses. Primitivo is not happy, and continues to say so for the rest of the afternoon.
That evening, Primitivo is apparently on sentry duty – Robert Jordan goes to visit him (we don't see the visit).
The next day, Primitivo fights in Pilar's group, attacking the upper sentry post.
After taking the post, he carries the wounded Fernando back to the bridge with Agustín.